Ion sources

ABSTRACT

The ion source comprises a primary plasma generator constituted by a hot cathode and an anode, between which an arc is produced, and by an intermediate electrode enclosing the arc in proximity to an extraction orifice pierced in the anode. Magnetic field creating means are provided between the intermediate electrode and the extraction orifice, which has a relative small crosssection so that the pressure of the gases occurring inside the primary plasma generator is independent of the pressure existing downstream of the extraction orifice. This pressure downstream of extraction orifice is less than the pressure inside the primary plasma generator. On the downstream side there is an extraction electrode. A fifth electrode is arranged downstream of the extraction orifice of the anode and upstream of the extraction electrode and means are provided for applying to this fifth electrode a positive potential with respect to that of the anode and of adjustable value, which is small in absolute value with respect to the potential of the extraction electrode.

United States Patent Gautherin et a1.

1 1 June 17, 1975 1 1 [ON SOURCES [75] Inventors: Guy Gautherin, Orsay;Jean Aubert,

Vaucresson, both of France [73] Assignee: Agence National deValorisalion de la Recherche (ANVAR), Paris, France [22] Filed: Oct. 11,1972 [21] Appl. No.: 296,594

[30] Foreign Application Priority Data Oct, 13, 1971 France 71.36701[52] US. Cl. 315/111; 250/423; 313/63 [51] Int. Cl H01] 37/08; HOSh l/OO[58] Field of Search 250/423-424, 250/427; 313/63; 315/111 [56]References Cited UNITED STATES PATENTS 3,238,414 3/1966 Kelly et al315/111 3,265,918 8/1966 Wittkower 313/63 3,631,283 12/1971 Gautherin etal 313/63 Beams," Rev. of Scientific Instruments, Vol. 38, 4-1967, pp.467, 470-473, 476-477.

Primary Examiner-James W. Lawrence Assistant Examiner-Wm. H. PunterAttorney, Agent. or F irm- Flynn & Frishauf s7 1 ABSTRACT The ion sourcecomprises a primary plasma generator constituted by a hot cathode and ananode, between which an arc is produced, and by an intermediateelectrode enclosing the arc in proximity to an extraction orificepierced in the anode. Magnetic field creating means are provided betweenthe intermediate electrode and the extraction orifice, which has arelative small cross-section so that the pressure of the gases occurringinside the primary plasma generator is independent of the pressureexisting downstream of the extraction orifice. This pressure downstreamof extraction orifice is less than the pressure inside the primaryplasma generator. On the downstream side there is an extractionelectrode. A fifth electrode is arranged downstream of the extractionorifice of the anode and upstream of the extraction electrode and meansare provided for applying to this fifth electrode a positive potentialwith respect to that of the anode and of adjustable value, which issmall in absolute value with respect to the potential of the extractionelectrode.

10 Claims, 3 Drawing Figures ION SOURCES The present invention relatesto improvements in or to ion sources.

There is already known an ion source called a duoplasmatron" which isconstituted by a hot cathode, an intermediate electrode and an anodepierced by an extraction orifice.

In this known device, an arc is produced between the cathode and theanode and this arc is pinched in the vicinity of said extraction orificeby means of, on one hand, an electrostatic effect caused by theintermediate electrode and, on the other hand, by the effect of amagnetic lens whose poles are constituted by the anode and theintermediate electrode, means for the production of a magnetic fieldbeing provided to create such a field between the intermediate electrodeand the anode, this magnetic field having the axial direction, that isto say the direction of a line joining the hot cathode and theextraction orifice. There is obtained, at the outlet of this device, aplasma formed by a mixture of electrons and positive ions. In addition,in this duoplasmatron source, the extraction orifice has relativelysmall dimensions.

Improvements according to the present invention seek to increase to anappreciable extent, the ion current of the plasma obtained. The ionsource according to the invention enables, in addition, variation of theenergy of the electrons that it produces, to obtain not only positiveions but also negative ions and, the generation of a plasma composed ofneutral particles.

The ion source according to the invention is characterised by the factthat it comprises, in addition to the three usual electrodes of theduoplasmatron (that is to say a hot cathode, an intermediate electrodeand an anode pierced by an extraction orifice) constituting a primaryplasma generator, and the usual downstream extraction electrode a fifthelectrode arranged downstream of the extraction orifice of the anode butupstream of the extraction electrode and means for applying to the fifthelectrode a positive potential with respect to that of the anode, thesemeans being arranged so that said positive potential has an adjustablevalue, preferably higher than a critical valve, but much less inabsolute value than the potential of the extraction electrode.

The fourth electrode has advantageously the shape of an expansionchamber; in this case, this electrode can also comprise means forcreating magnetic induction in said chamber. The fifth electrode isadvantageously located adjacent the anode without an interveningexpansion space.

In a first embodiment of the invention, the expansion chamber comprisesan orifice for the introduction of gas or vapor intended to be ionisedby the primary plasma. In a second embodiment, said chamber comprisesheating means to vaporise solid substances introduced inside thischamber, the vaporised substances being also intended to be ionised.

Other features or characteristics according to the invention will appearalso in the course of the more detailed description which follows ofpreferred embodi ments according to the invention, with reference to thedrawings in which:

FIG. 1 illustrates a first embodiment of the invention,

FIG. 2 is a diagram which shows the intensity of the currents of ionsand of electrons obtained by the device according to the invention, as afunction of the voltage between the fourth electrode and the anode, and

FIG. 3 illustrates a second embodiment of the invention.

The ion source according to the invention comprises, firstly, in knownmanner, a primary plasma generator 1 (FIGS. 1 and 3) of theduoplasmatron type. This primary plasma generator is composed of a hotcathode 2, of an intermediate electrode 3 brought to a potential greaterthan that of the cathode 2 and an anode 4 pierced by an extractionorifice 5 and brought to a potential, generally that of ground, greaterthan that of the electrode 3. Magnetic field-creating means 3a areprovided to create a magnetic field between the intermediate electrodeand the extraction orifice. There is introduced into such a device,which is shown diagramatically in FIGS. 1 and 3, a gas for examplehydrogen and there is produced an arc between the cathode 2 and theanode 4. This generator 1 then emits a jet of plasma downstream of theextraction orifice 5, this jet of plasma being composed of electrons andprimary ions. The extraction orifice 5 has relatively re duceddimensions its cross-section being at the most, equal to 5 mm. Thelatter feature has the object of enabling the pressure p, of the gasesoccurring upstream of the anode to be uninfluenced by the pressure p(less than p of the gases occurring downstream of the anode; in otherwords, the gases downstream of the anode must not reach the cathode 2.

The ion source shown in FIG. 1 includes an extraction electrode 18 andalso comprises, in addition, according to the invention, a fifthelectrode 6 and means 7 for bringing this electrode 6 to a positivepotential with respect to the anode 4, whose value is adjustable.Preferably, this potential is greater than a critical value V Thepotential of the electrode 6 being positive with respect to that of theanode, there is hence obtained, at the outlet of the duoplasmatron,current of electrons.

Means 7 are composed, for example, as in the embodiment of the inventionshown in FIG. I, of a direct current source 8 in series with anadjustable resistance 9. In the embodiment of the invention shown inFIG. 3, said means 7 are composed of a direct current source 10 whosevoltage is adjustable.

The electrode 6 has, in general, the shape of a reservoir or expansionchamber and is arranged downstream and facing the expansion orifice 5 soas to receive the jet of plasma produced by the generator 1. In a firstembodiment of the invention, the reservoir formed by the electrode 6 isempty and, in another embodiment, said reservoir contains a gas or avapor. This gas or this vapor can be such that, if it were in contactwith the filament of the cathode 2, it would damage the latter.

The abovesaid value V, is a critical value for the voltage between theanode 4 and the electrode 6; when this voltage exceeds this value Vthere is produced a sudden increase in intensity, respectively, of theion current and, to a lesser degree of the electron current, at thelevel of the electrode 6. The inventors have, in fact, noted thatstarting from a certain positive value V of the potential differencebetween the electrode 6 and the anode 4, there is produced a newionisation or arc between the anode 4 and the electrode 6 manifested bythe abovesaid sudden increases in intensity. This phenomenon isillustrated by FIG. 2 which is a diagram in which there is indicated, asabscissae, the potential V applied between the electrode 6 and the anode4 and, as ordinates, the intensity I of the current of ions or ofelectrons at the level of the electrode 6. In this diagram, the curve11, in full line, shows the variations in the intensity of the ioncurrent (these ions being possibly. as will be seen below, positive ornegative) and the curve 12, in interrupted line, shows the variations inthe intensity of the current of electrons, these intensities being, ofcourse, measured at the level of the electrode 6. There is seen in thisFigure, besides the said sudden increases in the intensity from thevalue V that the intensity of the electron current obeys a linear law ofvariation as a function of the potential V This intensity of theelectron current increases linearly when the potential V increases,increases suddenly when this potential passes from a value below to avalue greater than V and then increases linearly when V has exceeded V,,with a growth rate substantially equal to that which appears when V isless than V,. It will also be noted that the intensity of the ioncurrent tends appreciably to approach a limiting value when thepotential V increases, above the value V It should be noted that thereis obtained a discharge of the diode type between the anode 4 and theelectrode 6.

As regards the values V there may be mentioned the two followingexamples, obtained in the course of ex periments carried out within thescope of the invention:

In the first experiment, the duoplasmatron or generator 1 containedargon, and the electrode 6, in the shape of a reservoir, also containedargon. In this case, the critical value V was found to be comprised,according to the conditions of the experiment, between 25 and 30 volts.

In the second experiment, the generator 1 contained argon whilst theelectrode 6, in the form of a reservoir, contained oxygen, it was thennoted that the critical voltage was around 50 volts and that thisvoltage V diminished when the pressure of oxygen in the electrode 6increased.

In general, the inventors noted that the critical voltage V depends,firstly, on the nature of the gases present in the generator 1 and,possibly in the electrode 6, secondly, on the conditions of discharge inthe generator l and on the pressure of the gas which is possiblypresent, in the electrode 6. It was found that V had a value comprisedbetween 5 and 100 volts.

Referring now, more particularly, to the embodiment of the inventionshown in FIG. 1, it is seen that the electrode 6 has substantially theshape of a cylinder of which the axis extends that of the duoplasmatronand provides an inlet aperture 13, facing the extraction orifree 5, andan outlet aperture 14 through which is injected the ion current andelectrons formed inside this electrode 6.

The electrode 6 which forms, in this case, an expansion chamber,comprises, in addition, means 15 for enabling the introduction of a gasor of a vapour inside the said chamber, this gas or this vapour beingintended to be ionised by the electrons and primary ions emerging fromthe duoplasmatron. The means 15 are, for example, constituted by anouter reservoir (not shown) and an intake pipe.

Moreover, there are provided means constituted by coils l6 and 17arranged, for example, on the outer periphery of the electrode 6 tocreate a magnetic induction inside the said chamber formed by theelectrode 6,

this induction having preferably a direction substantially parallel tothe axis of the cylinder formed by the electrode 6. The inventors have,in fact, noted that the presence of a magnetic induction inside theexpansion chamber increases the ionic and electronic intensities insidethe latter as well as the yield of ionisation in this chamber.

An extraction electrode I8, known in itself, is arranged downstream ofthe electrode 6. This electrode 18 enables, when it is suitablypolarised, the extraction, from the expansion chamber formed by theelectrode 6, of positive or negative ions which have arrived or beenformed in said electrode 6. The potential of this electrode 18 relativeto the electrodes of the source has, in a general way, a very high valuewith respect to the relative potential of the electrodes of the source.If the potential of the electrode 18 is positive, it will be thenegative ions and the electrons which will be extracted, whilst, if thepotential of this electrode 18 is negative, it will be the positive ionswhich will be extracted through this electrode. Since the potential ofthe electrode 18 relative to the nearest electrode (the electrode 6) ofthe source is high but may be positive or negative, while the potentialof the electrode 6 relative to that of the anode 4 is always positivebut is at most of the order of magnitude of volts, a statement comparingthese successive interelectrode potentials must take the form of sayingthat the potential of electrode 6 relative to electrode 4 is positiveand small in absolute value compared to the potential of electrode 18relative to electrode 6, the term absolute value being used in thecommon mathematical sense of the term, meaning magnitude regardless ofsign. In other words, such a statement means that the difference inpotential between electrodes 6 and 4 is small compared to the difference(regardless of sign) in potential between electrodes 18 and 6.

It is already known, from French Pat. No. 1,585,902 filed 9 Apr. I968for Device for producing intense ion beams, especially metallic," thatit is possible to create positive ions from a gas by means of a plasmajet emerging from the duoplasmatron. In the present case, the said gasis that which is contained in the chamber formed by the electrode 6.This ionisation has a double origin: on one hand, the electrons producedby the generator 1 ionised by shock, said gas by tearing off one orseveral electrons from each of its atoms A, which is expressed, in thecase of simple ionisation, by the following relationship:

A e A 2e and, in general:

A-l-e A" +(n+ l)e,

or again, in the case of a molecular gas AB:

AB e A B'" (m+n+l )e;

on the other hand, positive ions produced by the generator 1 exchangetheir charge with neutral atoms from the expansion chamber. Thisphenomenon is expressed by the following relationship, in the simplestcase:

A H A H;

of course, there could be a relationship of the type:

The inventors have noted that there could be created, by means of thebeam emerging from the duoplasmatron, not only positive ions but alsonegative ions with a single or multiple charge. This ionisation isproduced by the primary electrons generated by the generator l; in thecase of a molecular gas AB affected by these electrons, the ionisationis produced according to one and/or other of the following equations:

In the case of a monatomic gas A affected by electrons generated by thebeam produced by a duoplasmatron, the ionisation is effected accordingto the following relationship:

in which the expression hv expresses simply that the ionisation isradiative, that is to say that it produces an emission of light.

It will be noted that this production of negative ions can be achievedeven if the source does not have means 7 for applying an adjustablepotential to the chamber Of course, as has already been seen, accordingto the polarity of the extraction electrode 18, it will be positive ornegative ions which will be extracted from the chamber formed by theelectrode 6.

In the embodiment shown in FIG. 3, the electrode 6 also has the shape ofan expansion chamber but it is formed in the shape of an oven providedwith heating means 22 to vaporise a substance 19 which has beenintroduced into this oven. In this embodiment of the invention, thesource comprises, in addition, downstream of the electrode 6, after itsoutlet aperture 14, magnetic separating means 20 adapted to separate thetrajectory of primary ions from that of (positive or negative) ionsproduced in the expansion chamber. Moreover, immediately after themagnetic separating means 20, there is provided an electrode 21 broughtto a positive potential with respect to that of the electrode 6 andwhich is adapted to trap the electrons of the plasma beam emerging fromthe source. There is hence obtained, by means of this modification ofthe source according to the invention, a beam of positive or negativeions which is purified of electrons and primary ions.

In another modification of the invention there are used negative and/orpositive ions emitted by the source, to produce a neutral plasma. Itsuffices, for this, to send the beam of ions onto a gas target in whichthe ions, as the case may be, lose or gain one or several electrons andare neutralised whilst preserving their kinetic energy.

It will be noted that the ion source according to the invention enablesthe obtaining of intense ion beams from various materials without therebeing any risk of contamination of the filament of the hot cathode bythese materials.

Moreover, it will be noted that, when the electrode 6 has the shape ofan expansion chamber, the pressure inside the latter can be varied oncondition, of course, that this pressure remains less than that whichexists inside the primary generator 1.

The ion source which has just been described with respect to FIGS. 1 to3 can lend itself to numerous applications and there may be mentioned,by way of example, application of ions for doping semi-conductormaterials. Thus, in the case of silicon, there have already beenimplanted positive ions (acceptors):l3 (boron), P (phosphorus) and 0*(oxygen), and negative ions (donors): Fe (iron), Si (silicon) and O. Theion source according to the invention can also be used for a massspectrometer and a VAN-DE-GRAAFF type accelerator.

As is self-evident and as emerges already from the foregoing, theinvention is in no way limited to those of its types of application, norto those embodiments of its various parts, which have been moreespecially indicated; it encompasses, on the contrary, allmodifications.

We claim:

1. Ion source comprising a primary plasma generator constituted by a hotcathode and an anode, between which an arc is produced, said anodedefining an extraction orifice, and by an intermediate electrodepinching the arc in proximity to said extraction orifice in the anode,magnetic field producing means creating a magnetic field between theintermediate electrode and the extraction orifice, an extractionelectrode downstream of said anode, said extraction orifice of the anodehaving a relatively small cross-section so that the pressure of thegases occurring inside said primary plasma generator is independent ofthe pressure existing downstream of said extraction orifice, saidpressure downstream of the extraction orifice being less than saidpressure inside the primary plasma generator, said generator comprisinga fifth electrode arranged downstream of the. extraction orifice of theanode and upstream of the extraction electrode and means for applying tosaid fifth electrode a potential positive with respect to that of theanode and of adjustable value, said positive potential being small inabsolute value compared to the potential of the extraction electrodewith respect to that of said fifth electrode.

2. Ion source according to claim 1, wherein the crosssection of saidextraction orifice is less than 5 mm*.

3. Ion source according to claim 2, wherein the fifth electrode is inthe form of an enclosure including an inlet aperture, arranged adjacentthe extraction orifice without intervening expansion space but leavingan insulation space, and an outlet aperture.

4. Ion source according to claim 1, wherein the fifth electrode is inthe form of an enclosure including an inlet aperture, arranged adjacentthe extraction orifice without an intervening expansion space butleaving an insulation space, and an outlet aperture.

5. Ion source according to claim 4, comprising means for creatingmagnetic induction inside said enclosure.

6. Ion source according to claim 4, comprising means arranged downstreamof said outlet aperture for separating secondary ions formed in saidenclosure from the primary ions formed in said primary plasma generator.

7. [on source according to claim 1, comprising means arranged downstreamof the fifth electrode, for separating the electrons from the ionsproduced by said source.

8. Ion source comprising a primary plasma generator constituted by a hotcathode and an anode, between which an arc is produced, said anodedefining an extraction orifice, and by an intermediate electrodepinching the arc in proximity to said extraction orifice in the anode,magnetic field producing means creating a magnetic field between theintermediate electrode and the extraction orifice, said extractionorifice having a relatively small cross-section so that the pressure ofthe gases occurring inside said primary plasma generator is independentof the pressure existing downstream of said extraction orifice, saidpressure downstream of the extraction orifice being less than saidpressure inside the primary plasma generator. said generator comprisingan additional electrode arranged downstream of said extraction orificeof the anode. said additional electrode being in the form of anenclosure including an inlet aperture arranged adjacent the extractionorifice without an intervening expansion space but leaving an insulationspace, and an outlet aperture, means for applying to said additionalelectrode a potential posi tive with respect to that of the anode and ofadjustable value, and means for introducing a gas or a vapor into saidenclosure.

9. Ion source comprising a primary plasma generator constituted by a hotcathode and an anode, between which an arc is produced, said anodedefining an extraction orifice, and by an intermediate electrodepinching the arc in proximity to said extraction orifice in the anode,magnetic field producing means creating a magnetic field between theintermediate electrode and the extraction orifice, said extractionorifice having a relatively small cross-section so that the pressure ofthe gases occurring inside said primary plasma generator is independentof the pressure existing downstream of said extraction orifice, saidpressure downstream of the extraction orifice being less than saidpressure inside the primary plasma generator, said generator comprisingan additional electrode arranged downstream of said extraction orificeof the anode, said additional electrode being in the form of anenclosure including an inlet aperture arranged adjacent the extractionorifice without an intervening expansion space but leaving an insulationspace, and an outlet aperture, means for applying to said additionalelectrode a potential positive with respect to that of the anode and ofadjustable value, and heating means for vaporizing a solid substancepresent inside said enclosure.

10. Ion source comprising a primary plasma generator constituted by ahot cathode and an anode, between which an arc is produced, said anodedefining an extraction orifice, and by an intermediate electrodepinching the arc in proximity to said extraction orifice in the anode,magnetic field producing means creating a magnetic field between theintermediate electrode and the extraction orifice, said extractionorifice having a relatively small cross-section so that the pressure ofthe gases occurring inside said primary plasma generator is independentof the pressure existing downstream of said extraction orifice, saidpressure downstream of the extraction orifice being less than saidpressure inside the primary plasma generator, said generator comprisingan additional electrode arranged downstream of said extraction orificeof the anode, said additional electrode being in the form of anenclosure including an inlet aperture arranged adjacent the extractionorifice without an intervening expansion space but leaving an insulationspace, and an outlet aperture, means for applying to said additionalelectrode a potential positive with respect to that of the anode and ofadjustable value, and means for creating magnetic induction inside saidenclosure.

1. Ion source comprising a primary plasma generator constituted by a hotcathode and an anode, between which an arc is produced, said anodedefining an extraction orifice, and by an intermediate electrodepinching the arc in proximity to said extraction orifice in the anode,magnetic field producing means creating a magnetic field between theintermediate electrode and the extraction orifice, an extractionElectrode downstream of said anode, said extraction orifice of the anodehaving a relatively small cross-section so that the pressure of thegases occurring inside said primary plasma generator is independent ofthe pressure existing downstream of said extraction orifice, saidpressure downstream of the extraction orifice being less than saidpressure inside the primary plasma generator, said generator comprisinga fifth electrode arranged downstream of the extraction orifice of theanode and upstream of the extraction electrode and means for applying tosaid fifth electrode a potential positive with respect to that of theanode and of adjustable value, said positive potential being small inabsolute value compared to the potential of the extraction electrodewith respect to that of said fifth electrode.
 2. Ion source according toclaim 1, wherein the cross-section of said extraction orifice is lessthan 5 mm2.
 3. Ion source according to claim 2, wherein the fifthelectrode is in the form of an enclosure including an inlet aperture,arranged adjacent the extraction orifice without intervening expansionspace but leaving an insulation space, and an outlet aperture.
 4. Ionsource according to claim 1, wherein the fifth electrode is in the formof an enclosure including an inlet aperture, arranged adjacent theextraction orifice without an intervening expansion space but leaving aninsulation space, and an outlet aperture.
 5. Ion source according toclaim 4, comprising means for creating magnetic induction inside saidenclosure.
 6. Ion source according to claim 4, comprising means arrangeddownstream of said outlet aperture for separating secondary ions formedin said enclosure from the primary ions formed in said primary plasmagenerator.
 7. Ion source according to claim 1, comprising means arrangeddownstream of the fifth electrode, for separating the electrons from theions produced by said source.
 8. Ion source comprising a primary plasmagenerator constituted by a hot cathode and an anode, between which anarc is produced, said anode defining an extraction orifice, and by anintermediate electrode pinching the arc in proximity to said extractionorifice in the anode, magnetic field producing means creating a magneticfield between the intermediate electrode and the extraction orifice,said extraction orifice having a relatively small cross-section so thatthe pressure of the gases occurring inside said primary plasma generatoris independent of the pressure existing downstream of said extractionorifice, said pressure downstream of the extraction orifice being lessthan said pressure inside the primary plasma generator, said generatorcomprising an additional electrode arranged downstream of saidextraction orifice of the anode, said additional electrode being in theform of an enclosure including an inlet aperture arranged adjacent theextraction orifice without an intervening expansion space but leaving aninsulation space, and an outlet aperture, means for applying to saidadditional electrode a potential positive with respect to that of theanode and of adjustable value, and means for introducing a gas or avapor into said enclosure.
 9. Ion source comprising a primary plasmagenerator constituted by a hot cathode and an anode, between which anarc is produced, said anode defining an extraction orifice, and by anintermediate electrode pinching the arc in proximity to said extractionorifice in the anode, magnetic field producing means creating a magneticfield between the intermediate electrode and the extraction orifice,said extraction orifice having a relatively small cross-section so thatthe pressure of the gases occurring inside said primary plasma generatoris independent of the pressure existing downstream of said extractionorifice, said pressure downstream of the extraction orifice being lessthan said pressure inside the primary plasma generator, said generatorcomprising an additional electrode arranged downstream of saiDextraction orifice of the anode, said additional electrode being in theform of an enclosure including an inlet aperture arranged adjacent theextraction orifice without an intervening expansion space but leaving aninsulation space, and an outlet aperture, means for applying to saidadditional electrode a potential positive with respect to that of theanode and of adjustable value, and heating means for vaporizing a solidsubstance present inside said enclosure.
 10. Ion source comprising aprimary plasma generator constituted by a hot cathode and an anode,between which an arc is produced, said anode defining an extractionorifice, and by an intermediate electrode pinching the arc in proximityto said extraction orifice in the anode, magnetic field producing meanscreating a magnetic field between the intermediate electrode and theextraction orifice, said extraction orifice having a relatively smallcross-section so that the pressure of the gases occurring inside saidprimary plasma generator is independent of the pressure existingdownstream of said extraction orifice, said pressure downstream of theextraction orifice being less than said pressure inside the primaryplasma generator, said generator comprising an additional electrodearranged downstream of said extraction orifice of the anode, saidadditional electrode being in the form of an enclosure including aninlet aperture arranged adjacent the extraction orifice without anintervening expansion space but leaving an insulation space, and anoutlet aperture, means for applying to said additional electrode apotential positive with respect to that of the anode and of adjustablevalue, and means for creating magnetic induction inside said enclosure.